1. Field of the Invention
This invention relates to methods and apparatus for producing sintered products and, more specifically, to methods and apparatus for producing nuclear fuel pellets of generally uniform porosity, and the like.
2. Description of the Prior Art
A number of techniques have been developed for manufacturing nuclear fuel pellets, but all of these techniques have failed to provide a consistently satisfactory product. In this respect, it is important to understand the unique and essentially hostile environment in which these pellets are used. Basically, individual pellets of uranium dioxide, or other nuclear fuel pellets, are loaded into and sealed in a slender, hollow metal tube. This fuel rod, or "fuel pin", is one of an array of similar rods that establish in accumulation of fissonable material in sufficient concentration to support sustained fission reactions within the core of a nuclear reactor.
These fission reactions generate heat, which can be used to produce useful power; neutrons, which are the nuclear particles that initiate and sustain the fission reactions; and fission products, which usually are radioactive and must be contained within the individual fuel rods for reasons of health and safety. Thus, the fissionable material nuclei, on absorbing a neutron, split more or less in half, the halves thereby forming the nuclei of two entirely different elements, some of which are in the gaseous state under the temperature and pressure conditions within the fuel rod. In these circumstances of high temperature and the formation of elements with different chemical and physical properties within the pellets the need to maintain the integrity of the fuel rod is an extremely difficult task.
Perhaps the most significant source of difficulty in this respect is caused by the structural instability of the fuel pellets. Fuel pellets, for example, under conditions frequently encountered within reactor cores tend to "swell", or to undergo a change in physical dimensions. The cause, or causes, of this swelling is not fully understood, but the phenomenon nevertheless has been observed many times. Clearly, given the usual reactor core environment, even relatively minor changes in pellet dimensions can cause the enclosing metal tube to burst or to collapse and release radioactive fission products.
Radioactive gas generated within the pellets also must be allowed to escape into a void space within the rod structure, to help preserve the physical integrity of the individual pellets.
A number of pellet manufacturing methods have been developed through the years. These techniques reflect different attempts to overcome the effects of fuel pellet swelling. Basically, all of these proposed solutions appear to have been directed to the preparation of a suitably low density fuel pellet, characterized by a controlled and uniformly distributed porosity. Ideally, these low density pellets should accommodate fuel swelling under arduous reactor condition of pressure, temperature and radiation. These pellet preparation techniques, however, are not entirely satisfactory.
In some instances, a volatile additive is combined with the powdered nuclear fuel. On heating to enable the powered fuel so sinter or coalesce into one mass, the volatile matter should evaporate, and, in this manner, produce a strong but porous pellet structure. Unfortunately, this volatilization technique frequently leaves an undesirable residue within the pores, or an unsatisfactorily small void volume.
There are other techniques, for example, which rely on the method of fuel powder granulation, or make use of blended fuel powders in which each of the constituent powders in the blend is heat treated at a different temperature before being mixed together and sintered. In the granulation method the resulting pores are not evenly distributed through the pellet volume but tend to appear as a few undesirably large pores. The heat-treated powder, moreover, imposes difficult production control requirements.